An optical transmitter module is an important component in networking systems. The purpose of an optical transmitter module is to convert data signals in electrical form into corresponding data signals in optical form. In this manner, the data can be communicated as light to another module (e.g., an optical receiver module) through a light-conducting medium, such as a fiber optic cable.
The optical transmitter module typically employs a laser to convert the electrical data signals into the light data signals. One commonly utilized semiconductor laser is the vertical cavity surface emitting laser (VCSEL). However, the VCSEL is configured to operate only with input signals (e.g., drive waveforms) that conform to particular predetermined electrical properties. The drive waveforms can have both dc operating parameters and ac operating parameters. For example, the dc operating parameters may include bias current to obtain either average or low state output power. The ac operating parameters may include modulation current, peaking current, and time constant parameters associated with pulsed waveforms. The data signals typically do not have these predetermined electrical characteristics (e.g., specific dc and ac operating parameters). Consequently, a circuit is needed for accepting the data signals, and responsive thereto, for generating corresponding VCSEL drive signals (e.g., a drive waveform) with the electrical characteristics that are suitable to drive the VCSEL. This circuit is commonly referred to as a VCSEL driver.
Furthermore, the VCSEL driver programs or sets the drive waveform with particular dc and ac parameters in order to optimize the bit error rate (BER) of the fiber optic link using the transmitter. The bit error rate is simply a measure of the number of bit errors caused by the transmitter module. A bit error is simply a data error when a data “1” is transmitted as a data “0” or when a data “0” is transmitted as a data “1”.
There are two main approaches in the design of prior art laser drivers. The first approach employs a closed loop (i.e., uses optical feedback to adjust the light output power) to program the drive waveforms. The second approach employs an open loop (i.e., does not use optical feedback to adjust the light output power) to program the drive waveforms. These prior art approaches with their attendant disadvantages are described hereinafter.
Closed-Loop Approaches
U.S. Pat. No. 5,638,390 describes an exemplary closed-loop approach embodied in a laser output power stabilizing circuit. The laser output power stabilizing circuit uses a photodiode to monitor the laser's optical power. The photodiode output is compared to a reference voltage from a digital potentiometer, to obtain the correct dc bias current for the laser. At the time of the transmitter's manufacture, the digital potentiometer is set to optimize the laser's dc bias current. During operation of the transmitter, the laser's bias current is adjusted when any change in photodiode output occurs.
Unfortunately, these closed-loop approaches suffer from several disadvantages. First, the use of the photodiode increases the cost of the optical transmitter. Second, the requirement of the photodiode introduces packaging concerns related to the mounting of the photodiodes in such a manner as to be optimally aligned with the VCSEL. Third, the closed-loop approaches require complex feedback circuits that need to be replicated for each VCSEL, thereby further increasing costs and manufacturing complexity.
Open-Loop Approaches
The data sheet for the AMCC S7011 transmitter integrated circuit (IC) that is available from Applied Micro Circuits Corporation (AMCC) describes an exemplary open-loop approach. The S7011 IC appears to be capable of adjusting the laser drive waveform parameters Imod and Ibias, given input from an external source (e.g., a microprocessor), or input from external resistors and voltage references. Unfortunately, the prior art open-loop approaches, including the AMCC approach, fail to provide or provide very limited mechanisms to adjust the drive waveform based on changes in age and temperature of the laser. These prior art open-loop approaches also fail to allow programming of the transitional aspects of the VCSEL drive waveform (e.g. negative peaking).
VCSEL Arrays
Recently, there has been interest in moving from a single VCSEL to an array of VCSELs, which for example, can be a plurality of VCSELs that are arranged in a row. As can be appreciated, an array of VCSELs can be employed to transmit more data through multi-channel fiber optic cable than a single VCSEL can transmit through a fiber optic cable having a single channel. Unfortunately, one of the engineering challenges for implementing the array of VCSELS is that optical waveform uniformity across the VCSEL array needs to be maintained in order to optimize the BER of the fiber optic link.
Consequently, correct settings for the dc and ac parameters of the drive waveforms are particularly critical for fiber optic transmitters using an array of VCSELs. The parameters must be set to maintain optical waveform uniformity across the VCSEL array. The setting of these properties needs to occur at the beginning of operation and also at periodic intervals during the product's lifetime.
Semiconductor electrical to optical transmitters often require a scheme to program the optical de and ac operating characteristics of the light-emitting device. Preferably, the programming is performed at the beginning of product use, and periodically programmed throughout the lifetime of the transmitter. Unfortunately, the prior art approaches that do periodically program the waveforms during the lifetime of the transmitter are costly, complex to implement, and limited to dc parameters. Those prior art approaches that address some of the ac issues, such as modulation current, are limited to programming only at the beginning of product use. Consequently, if the product requires programming during the operating life of the driver, these prior art approaches are unable to perform this type of programming.
Age Dependence of Light Output
Ideally, the laser's performance in terms of light output remains constant throughout the operating life of the laser. If this were the case, the drive waveforms can be programmed once by the laser driver and would require no further changes or re-programming. Unfortunately, in reality, VCSEL light output tends to degrade over the operating life of the laser. Consequently, it would be desirable to have a mechanism in the VCSEL driver for periodically adjusting the VCSEL drive waveform parameters to compensate for the degradation. Regrettably, the prior art approaches that employ an open-loop approach, such as the AMCC approach, are limited to programming the waveform parameters at the beginning of the product life and do not have a mechanism for periodically adjusting the VCSEL drive waveform parameters to compensate for the degradation.
Temperature Dependence of Light Output
Moreover, in an ideal situation, the laser's light output would be independent of operating temperature. If this were the case, the drive waveform would not require adjustment as the operating temperature changes. Unfortunately, in reality, the laser's light output is dependent on operating temperature. Accordingly, it would be desirable to have a mechanism that adjusts the drive waveforms as the operating temperature changes. By so doing, optimum VCSEL optical waveform characteristics can be maintained. Regrettably, the prior art approaches do not offer any mechanism for periodically adjusting the VCSEL drive waveform parameters to compensate for changing operating temperatures.
Based on the foregoing, there remains a need for a digital control method and apparatus for semiconductor lasers that overcomes the disadvantages set forth previously.